You may end up needing to transcode the phone footage, but a couple of things to try ahead of that: Clear your media cache. If you are exporting straight to H264, use Software Encoding and not Hardware Encoding.
If that doesn't work you may need to transcode the iPhone clip(s) before bringing it into Premiere. Phone footage is quite problematic in editing software. You can use something like Shutter Encoder to do the transcoding.